For the formation of a coating film on the bodies of vehicles such as automobiles, motorcycles, container vehicles and the like, a method is widely used which comprises applying a thermosetting coating composition that contains a resin having thermosetting functional groups, such as a hydroxyl-containing acrylic resin, and a crosslinking agent, such as a melamine resin, to the vehicle body to be coated, and thereafter curing the applied coating composition by heating. This method can form a coating film having excellent film properties in terms of adhesion, hardness, etc.
In recent years, the above-mentioned coating film formation method has been required to reduce energy consumption and increase productivity. For example, the heat-curing step on an automobile body coating line is usually conducted at about 140° C. for about 40 minutes; when the conveyor runs at a speed of 3 m/min, the step requires a space for the drying furnace line about 120 m in length. Therefore, in order to save space and energy, shortening of the heating time is demanded in the heat-curing step. Moreover, the reduction of the heating time is preferable for decreasing CO2, soot, etc., which cause environmental problems.
In addition, the coating film formed on an automobile body is now increasingly required to have improved resistance to marring by car washes, scratching around keyholes, etc. in view of better appearance. It has thus become important to develop a coating composition that can form a coating film on an automobile body or the like, having excellent resistance to marring by car washes, scratching, etc.
To shorten the heating time in the formation of a coating film, Japanese Unexamined Patent Publications No. 1988-113085 and No. 1989-11169 disclose a method of coating a substrate with an ultraviolet-curable and thermosetting coating composition containing ultraviolet-curable polyfunctional (meth)acrylates, polyhydric alcohol mono(meth)acrylate polymers, and polyisocyanate compounds or melamine resins, etc., and then carrying out ultraviolet irradiation, followed by about 5 to about 30 minutes of heat-curing to form a coating film. However, although the method using this coating composition can shorten the heating time, the method cannot form a cured coating film having excellent mar resistance.
U.S. Pat. No. 5,425,970 discloses a coating film formation method that comprises forming a colored base coat on a substrate to be coated, coating the base coat with a thermosetting clear coating composition, heat-curing the resulting coating film, further coating the film with a radiation-curable clear coating composition containing radical-polymerizable binders such as (meth)acrylic-functional (meth)acrylic copolymers, epoxy resin (meth)acrylates, polyester (meth)acrylates, etc. or cationically polymerizable binders such as polyfunctional epoxy oligomers, etc. and irradiating the film to cure it. However, the method using this radiation-curable clear coating composition, although capable of reducing the total heating time, cannot form a cured coating film having excellent mar resistance.